# std::ranges::views::repeat, std::ranges::repeat_view

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< cpp‎ | ranges

Defined in header `<ranges>` |  |
template< std::move_constructible W, std::semiregular Bound = std::unreachable_sentinel_t > requires (std::is_object_v<W> && std::same_as<W, std::remove_cv_t<W>> && (/*integer-like-with-usable-difference-type*/<Bound> || std::same_as<Bound, std::unreachable_sentinel_t>)) class repeat_view : public ranges::view_interface<repeat_view<W, Bound>> | (1) | (since C++23)
namespace views { inline constexpr /* unspecified */ repeat = /* unspecified */; } | (2) | (since C++23)
Call signature |  |
template< class W > requires /* see below */ constexpr /* see below */ repeat( W&& value ); |  | (since C++23)
template< class W, class Bound > requires /* see below */ constexpr /* see below */ repeat( W&& value, Bound&& bound ); |  | (since C++23)
Helper concepts |  |
concept /*integer-like-with-usable-difference-type*/ = /*is-signed-integer-like*/<T> || (/*is-integer-like*/ <T> && std::weakly_incrementable<T>) | (3) | (exposition only*)

1) A range factory that generates a sequence of elements by repeatedly producing the same value. Can be either bounded or unbounded (infinite).
2) views::repeat(e) and views::repeat(e, f) are expression-equivalent to repeat_view<std::decay_t<decltype((E))>>(e) and repeat_view(e, f) respectively for any suitable subexpressions e and f.
3) Determines whether a type is integer-like and has a usable difference type.

`repeat_view` models `random_access_range`. If `Bound` is not std::unreachable_sentinel_t, `repeat_view` also models `sized_range` and `common_range`.

### Customization point objects

The name `views::repeat` denotes a customization point object, which is a const function object of a literal `semiregular` class type. For exposition purposes, the cv-unqualified version of its type is denoted as `__repeat_fn`.

All instances of `__repeat_fn` are equal. The effects of invoking different instances of type `__repeat_fn` on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, `views::repeat` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval<Args>()... meet the requirements for arguments to `views::repeat` above, `__repeat_fn` models

- std::invocable<__repeat_fn, Args...>,
- std::invocable<const __repeat_fn, Args...>,
- std::invocable<__repeat_fn&, Args...>, and
- std::invocable<const __repeat_fn&, Args...>.

Otherwise, no function call operator of `__repeat_fn` participates in overload resolution.

### Data members

Member | Definition
`movable-box`﻿<W> `value_` | the repeating element of the view (exposition-only member object*)
`Bound` `bound_` | the sentinel value (exposition-only member object*)

### Member functions

(constructor) | creates a `repeat_view` (public member function)
begin | obtains the beginning iterator of a `repeat_view` (public member function)
end | obtains the sentinel denoting the end of a `repeat_view` (public member function)
size | obtains the size of a `repeat_view` if it is sized (public member function)
##### Inherited from std::ranges::view_interface
empty | returns whether the derived view is empty. Provided if it satisfies `sized_range` or `forward_range`. (public member function of `std::ranges::view_interface<D>`)
cbegin (C++23) | returns a constant iterator to the beginning of the range. (public member function of `std::ranges::view_interface<D>`)
cend (C++23) | returns a sentinel for the constant iterator of the range. (public member function of `std::ranges::view_interface<D>`)
operator bool | returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of `std::ranges::view_interface<D>`)
front | returns the first element in the derived view. Provided if it satisfies `forward_range`. (public member function of `std::ranges::view_interface<D>`)
back | returns the last element in the derived view. Provided if it satisfies `bidirectional_range` and `common_range`. (public member function of `std::ranges::view_interface<D>`)
operator[] | returns the `n`th element in the derived view. Provided if it satisfies `random_access_range`. (public member function of `std::ranges::view_interface<D>`)

## std::ranges::repeat_view::repeat_view

repeat_view() requires std::default_initializable<W> = default; | (1) | (since C++23)
constexpr explicit repeat_view( const W& value, Bound bound = Bound() ); | (2) | (since C++23)
constexpr explicit repeat_view( W&& value, Bound bound = Bound() ); | (3) | (since C++23)
template < class... WArgs, class... BoundArgs > requires std::constructible_from<W, WArgs...> && std::constructible_from<Bound, BoundArgs...> constexpr explicit repeat( std::piecewise_construct_t, std::tuple<WArgs...> value_args, std::tuple<BoundArgs...> bound_args = std::tuple<>{} ); | (4) | (since C++23)

1) Default-initializes `value_` and value-initializes `bound_`﻿.
2) Initializes `value_` with value and initializes `bound_` with bound.
If `Bound` is not std::unreachable_sentinel_t and bool(bound >= 0) is false, the behavior is undefined.
3) Initializes `value_` with std::move(value) and initializes `bound_` with bound.
If `Bound` is not std::unreachable_sentinel_t and bool(bound >= 0) is false, the behavior is undefined.
4) Initializes `value_` with std::make_from_tuple<T>(std::move(value_args)) and `bound_` with std::make_from_tuple<Bound>(std::move(bound_args)).
If `Bound` is not std::unreachable_sentinel_t and bool(bound >= 0) is false, the behavior is undefined.

### Parameters

value | - | the value to be repeatedly produced
bound | - | the bound
value_args | - | the tuple containing the initializers of `value_`
bound_args | - | the tuple containing the initializers of `bound_`

## std::ranges::repeat_view::begin

constexpr /*iterator*/ begin() const; |  | (since C++23)

Returns `iterator`﻿(std::addressof(*`value_`﻿)).

## std::ranges::repeat_view::end

constexpr /*iterator*/ end() const requires (!std::same_as<Bound, std::unreachable_sentinel_t>); | (1) | (since C++23)
constexpr std::unreachable_sentinel_t end() const; | (2) | (since C++23)

1) Returns `iterator`﻿(std::addressof(*`value_`﻿),`` `bound_`﻿).
2) Returns std::unreachable_sentinel.

## std::ranges::repeat_view::size

constexpr auto size() const requires (!std::same_as<Bound, std::unreachable_sentinel_t>); |  | (since C++23)

Returns `to-unsigned-like`﻿(`bound_`﻿).

### Deduction guides

template< class W, class Bound = std::unreachable_sentinel_t > repeat_view( W, Bound = Bound() ) -> repeat_view<W, Bound>; |  | (since C++23)

### Nested classes

iterator | the iterator type (exposition-only member class*)

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_ranges_repeat` | `202207L` | (C++23) | `std::ranges::repeat_view`

### Example

Run this code

    #include <iostream>
    #include <ranges>
    #include <string_view>
    using namespace std::literals;

    int main()
    {
        // bounded overload
        for (auto s : std::views::repeat("C++"sv, 3))
            std::cout << s << ' ';
        std::cout << '\n';

        // unbounded overload
        for (auto s : std::views::repeat("I know that you know that"sv)
                    | std::views::take(3))
            std::cout << s << ' ';
        std::cout << "...\n";
    }

Output:

    C++ C++ C++
    I know that you know that I know that you know that I know that you know that ...

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 4053 | C++20 | unary calls to `views::repeat` did not decay the argument | decay the argument
LWG 4054 | C++20 | calling `views::repeat` with a `repeat_view` did not create a nested `repeat_view` | creates a nested `repeat_view`

### See also

ranges::iota_viewviews::iota (C++20) | a `view` consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object)

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